There is a device shown in Patent Document 1, which prevents the fueling of non-predetermined types of fuels by the fueling nozzle which does not reach the predetermined diameter, i.e., a fueling error, with a mechanical structure. In such device, a flap which becomes a valve body is rotatably attached to one of a pair of reception means which can be pressed apart outwardly, and in a state before a pair of the reception means is pressed apart, a free end of the flap is engaged with the other of a pair of the reception means so as to block a rotation of the flap in a valve-opened position. Then, in such device, when a fueling nozzle, having a thickness allowing a pair of the reception means to be separated for a size releasing the aforementioned engagement, i.e., a thickness allowing a pair of the reception means to be pressed apart, is inserted, the rotation of the flap in the valve-opened position is allowed. Hence, even when only one of a pair of the reception means is moved outward by the inserted fueling nozzle, such device has a structure allowing the aforementioned engagement to be released.